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SMT Component Shifting

Question: We have a problem with chip components shifting during the reflow process, which is being detected during automatic optical inspection. The shifting is not limited to a single component, but rather changes daily, where the component shifting occurs and moves from one location to another very randomly.

What is the likely cause of this problem and are there ways we can overcome it?

Answer: Most times this is related to the components floating on the molten solder and the equipment having some vibrations, which could impact the locational position of those components. The vibration in a reflow oven may not be felt by the people in the area, but could be within the machine, or caused by a truck going by outside the building or a piece of equipment moving around within the building. Although the components are moving from their placed position, there was no mention as to whether they moved to a position where they were not acceptable, such as defined in J-STD-001 for Class 2 and 3 assemblies. If the movement did not cause them to be rejectable, then there is nothing to worry about. If the movement results in a defect condition, then further steps need to be taken to address this process problem.

The other issues to consider are the amount of solder paste deposited on the pads and the size of the pads or land areas. If the pads or land areas are too large, then excess solder will be deposited, which will allow the component to float and move around, so these condition need to be checked. The paste deposition should be around 6 mils or less for small chip component and the pads just slightly larger than the component termination.

There are lots of unknowns in the question, so I’ve answered it as generally as can be answer with the information provided.